Error in amplification factors

Focus: the error measure $A-\Aex$ as function of $\Delta t$ (recall that $p=a\Delta t$): $$\begin{equation} A-\Aex = \left\lbrace\begin{array}{ll} \Oof{\Delta t^2}, & \hbox{Forward and Backward Euler},\\ \Oof{\Delta t^3}, & \hbox{Crank-Nicolson} \end{array}\right. \end{equation}$$